The present invention relates to a laminated composite radius filler, its method of manufacture, and its method of use, especially in making high performance, high quality lower cost, aerospace composite assemblies.
Failure of composites, especially in aerospace applications, often initiates in the resin in the radius filler (i.e., xe2x80x9cnoodlexe2x80x9d) that fills the interface between plies in laminated joints. Cracks in the radius filler can be formed into the composite during manufacture (e.g., improper tooling, improper handling of tools, or residual strain), can grow from voids that provide a nucleation site for crack growth, or can arise when structural loading overstresses the resin. Residual tensile strain is often designed into composites today, and arises from mismatch in the coefficient of thermal expansion between the radius filler and the surrounding structure, especially the reinforcing fibers, or from shrinkage of the resin that arises during cure of the composite.
Composite spars or ribs are made by binding two xe2x80x9cCxe2x80x9d or xe2x80x9cUxe2x80x9d channels together to form a web with flanges. The channels generally comprise a plurality of plies of fiber-reinforced resin, commonly in the form of prepreg. The fiber reinforcement might be unidirectional tape or woven fabric, and, most commonly, is carbon fiber or fiberglass. For aerospace structure, it normally is woven carbon fiber fabric. The fabric usually is not isotropic with respect to its reinforcement strength. It may be easier to stretch or to expand the fabric in its width rather than in its length. In the different plies, the fabric can be oriented in different directions, specified as an angle of rotation from a reference direction. That is, the orientation might be 0xc2x0 or +45xc2x0 or xe2x88x9245xc2x0 or 90xc2x0, although other orientations are sometimes used. Here, xe2x80x9c+45xc2x0xe2x80x9d might mean that the fabric is rotated 45xc2x0 clockwise while xe2x80x9cxe2x88x9245xc2x0xe2x80x9d might mean a 45xc2x0 rotation in the counterclockwise sense. Ply orientation in the laminate evens the strength or impact resistance making the composite more uniform or less angle dependent. Oriented composites may be extremely strong in the direction of unidirectional reinforcing fibers while being relatively weak perpendicular to those fibers.
The plies are bent in a predetermined radius to form the xe2x80x9cCxe2x80x9d or xe2x80x9cUxe2x80x9d channel. When the channels are joined at the webs, a dimple occurs along the flange because of these radii. A radius filler fills the dimple. (See FIG. 1 or 11.) Using a radius filler prevents distortion that otherwise would occur when the spar or rib were loaded with a bending or twisting moment. Distortion can reduce the strength of the composite significantly and can also increase part variability (i.e., the spars simply are not the same shape from part to part).
Existing designs for radius fillers have produced fillers that are structurally inadequate, that are challenging and expensive to produce, or that leave the structural integrity of the resulting composite in question. Such designs often force post-manufacturing, non-destructive evaluation (NDE) and inspection (NDI), which slows production flow, increases cycle time, and increases cost. Therefore, there is a need for an improved radius filler that is easy and inexpensive to manufacture and structural sound to prevent distortion. The radius filler of the present invention allows the production of stronger, higher quality composites with lower variability while improving flow and cycle time and simultaneously reducing the overall composite cost.
When cracks cannot be avoided through a robust design as now achievable with the radius filler of the present invention, the structure needs to be made larger and heavier than optimal to withstand the design loads. Performance or payload in the aircraft is diminished because of the larger, heavier parts. Higher costs are also incurred both in its production and use.
Designers would like to build parts where performance of the radius filler is challenged even more severely than in existing, fielded aircraft. That is, designers would like the radius fillers having increased structural properties to withstand even greater stresses and pull-off loads than is achievable today. Such a radius filler would allow higher performance wings to be built. Therefore, absolute strength of the composite assembly is important. The radius filler of the present invention provides higher absolute strengths than are achievable with existing radius fillers. Therefore, the radius filler of the present inventions expands the domain of acceptable composite designs that can be used to meet aerospace challenges.
A laminated composite radius filler xe2x80x9ca noodlexe2x80x9d of the present invention better meets the challenges faced with composite design by reducing the initiation of processing (manufacturing) induced cracks or premature cracking of composite assemblies, like a spar or skin-stiffener interface, under structural loading. That is, the xe2x80x9cnoodlexe2x80x9d no longer is the weakest link in the composite structure or, if it remains the weakest, it still has a higher absolute strength than previous radius fillers allowed.
The present invention relates to a laminated composite radius filler having higher resistance to distortion, to its method of manufacture, and to its method of use. The radius filler permits design and manufacture of composite structures, like spars, ribs, or skin/stiffener assemblies, that have higher resistance to distortion, higher absolute strength, increased specific strength (i.e., strength per unit weight), lower part variability, and lower production cost. The radius filler enables the manufacture of stronger while lighter composite structure, which enables improved wing or other airfoil design. Cracking failure in the radius filler is reduced and pull-off strength is increased.
The present invention relates to the radius filler, to its method of manufacture, to its method of use, and to products that use it. A preferred radius filler of the present invention has a laminated fiber body and a unidirectional tip. The laminated fiber body typically has two distinct sections that are trapezoidal in cross-section. The upper section, for example, may have 14 or 18 plies of IM7/5250-4 thin tape with xc2x145xc2x0 orientation (i.e., the plies alternate from having a xc2x145xc2x0 orientation relative to the X-axis and xe2x88x9245xc2x0 orientation). The lower section is made from the same material but has 10 plies. Typically the noodle is completed with three additional, base plies. The number of plies and sections are selected to configure the radius filler to the shape of the dimple. To simplify the discussion, this description will focus on substantially triangular radius filler for T-section joints (such as joint between a stiffener and a skin), but other configurations for the dimple and the radius filler are possible.